1. Field of the Invention
The present invention relates in general to a natural gas engine, and more specifically to an apparatus and method for operating a natural gas engine with in situ generation of an autoignition substance.
2. Description of the Related Art
Natural gas engines produced and operating around the world typically use a spark ignition source to ignite the natural gas in the engine cylinder. This technology limits the achievable performance of an engine using natural gas as the fuel source since the spark does not volumetrically ignite the natural gas. There are typically regions of "too rich" and "too lean" gas mixtures within the cylinder. This causes incomplete combustion and inefficiency. The result is unburned gases in the exhaust stream, and this can mean that the engine efficiency has been reduced. In addition, the regions of "too lean" gas mixture may cause production of nitrogen oxides (NO.sub.x), sulfur oxides (SO.sub.x), and other pollutants. Attempts to increase the brake mean effective pressure (BMEP) in natural gas engines with spark ignition are limited by considerations of engine knock as well as NO.sub.x emissions. Also, increasing the cylinder pressure makes the spark ignition problem more difficult. Higher spark voltages are required and this leads to more tip erosion on the spark igniter.
It is known that an autoignition fuel can be used in a micropilot fuel injector in place of a spark source to cause better combustion of the natural gas in the engine cylinder. Previous work concentrated on the use of diesel fuel as the autoignition fuel. The quantity of diesel fuel required is typically between 0.1% and 5% of the quantity of natural gas on a volumetric basis.
It is believed that there are no commercially successful implementations of this concept, because of concerns with the performance of the micro pilot injectors as well as maintaining two separate sources of fuel, i.e., natural gas and diesel fuel.
It is also known that dimethyl ether (DME) has excellent autoignition properties. It has a cetane number higher than diesel fuel with similar hydrodynamic properties. From a thermophysical perspective, DME is similar to liquid natural gas (LNG) in that it is a gas at standard temperature and pressure. At room temperature, approximately six atmospheres (atm) of pressure are required to liquefy DME. Continuing research and development programs are underway in the United States, Europe, and Japan in attempting to develop engines and fuel systems that operate on 100% DME. These programs are directed to substituting DME for diesel fuel oil in diesel engines that operate on 100% diesel fuel.
PCT Application No. WO96/23755 is directed to a process for the preparation of fuel grade dimethyl ether from synthesis gas.
U.S. Pat. No. 4,417,000 relates to a process for making dimethyl ether from syngas.
Of interest is European Patent Application EP 0 761 942A1 which describes a process for generating power in a gas turbine cycle using dimethyl ether and/or methanol as the primary fuel.
There still exists a need for an apparatus and method that allows a diesel-type engine (diesel-type engine as used herein is intended to refer to a diesel cycle engine which does not rely on a spark ignition source as opposed to an Otto cycle engine which does employ a spark source for the cylinder). Preferably, the apparatus and method will generate an autoignition product in situ with a portion of the natural gas intended to be the primary fuel for the engine.